Ataxia telangiectasia (AT) is an inherited (recessive) neurologic disorder characterized by diverse pathological conditions including heightened sensitivity to ionizing radiation (I.R.) and cancer proneness. Even asymptomatic carriers of the AT gene (AT heterozygote) show a higher incidence of cancer. Recently, AT complementation groups A, C and D have been localized to chromosome 11q23, although the gene(s) at these loci, their products and, thus, their functions remain unidentified. Because cells from AT patients are more sensitive than are normal cells to killing by and show a greater incidence of chromosomal aberrations followings exposure to I.R., the primary defect in AT has been associated with faulty repair of I.R.-caused DNA damage. However, despite several studies, a defect in DNA repair in AT cells, after irradiation. Recent studies with yeast and other cells suggest that G2 arrest may play a crucial role in the response of cells to I.R. damage; resistant cells show longer delays than do sensitive cells. Cell-cycle delay in G2, prior to mitosis may increase the time for (DNA) damage assessment and repair. This proposal will investigate the hypothesis that, rather than a faulty DNA-damage repair process(es), AT cells lack control of DNA- damage repair, inducible during G2-arrest in irradiated normal but not in irradiated AT cells. Furthermore, involvement of a protein kinase C (PKC)-dependent pathway, both implicated in cell responses to I.R., in these processes and a defect of a pathway in AT cells will be investigated. The proposed study involves the use of well-characterized shuttle plasmid capable of replicating in human cells. I.R. -damaged plasmid will be transfected into and processed by irradiated or unirradiated AT and normal cells to assess DNA damage-repair capacity and fidelity. Progenies of the plasmid will be recovered and analyzed for reactivation (survival), mutation frequencies and, by gene sequencing, mutation types to determine whether differences in cell response to DNA- damage exist between AT and normal cells. By comparing mutagenesis of damage plasmid processed by irradiated and unirradiated cells, the effects of radiation on DNA, processing can be distinguished from those of specific mutations arising from DNA damage repair. I.R.-damage repair. To establish linkage of PKC activation, DNA repair, and G2 arrest, we will study the repair of I.R. damaged plasmid, cell survival and the degree of G2-arrest in cells with PKC activity blocked or stimulated by specific chemicals. Finally, differences between AT and normal cells in PKC activation and in the stimulation of steps, proximal to and distal to PKC in the pathway, by I.R. will be examined. The proposed studies of I.R. induced DNA-damage repair by AT cells and the roles of cell cycle and signal transduction via PKC in the repair process are designed to promote understanding of the molecular basis of the AT defect. Such an understanding will aid the development of effective screening methods and treatment for patients with this complex syndrome.